1. Technical Field
The present invention relates to an oscillation circuit, an oscillator, an electronic device, and a moving object.
2. Related Art
In recent years, oscillators including a serial interface have been used. In such oscillators, a method can be used for changing an output frequency, for example, by changing the setting of a multiplication number of a phase locked loop (PLL) by operating a register within the oscillator using a serial interface.
For example, “Si570 data sheet”, [online], Silicon Laboratories, [searched on Sep. 30, 2013], the Internet <URL:http://www.silabs.com/Support %20Documents/TechnicalDo cs/si570.pdf> discloses that it is possible to control a timing at which an output frequency is changed by storing a parameter for setting the output frequency in registers having addresses 14 to 18 and by writing predetermined data in registers having addresses 135 and 137 (see page 18 of the above-mentioned document).
Here, when an oscillator is used in a device constituting, for example, network synchronization, it is necessary to increase the modulation bandwidth. In the network synchronization, since it is necessary to make the clocks of respective devices conform to each other quickly and with a high level of accuracy, the oscillator constituting the network synchronization requires a high modulation bandwidth.
In order to increase the modulation bandwidth, a method using a parallel interface or an analog signal may also be used. However, when the parallel interface is used, the number of terminals of the oscillator increases, which is contrary to the demand for miniaturization. In addition, when the analog signal is used, problems such as frequency deviation based on a secular change in an analog device or a change in a characteristic due to temperature occur. Accordingly, it becomes difficult to make the clocks of the respective devices conform to each other with a high level of accuracy. Consequently, it is preferable that the modulation bandwidth be increased using a serial interface.
When the oscillator uses a serial interface, it is possible to increase the modulation bandwidth by reducing the amount of transmitted data necessary for a change in output frequency. For example, in the inventions disclosed in JP-A-05-284021 and JP-A-2006-5489, the amount of information transmitted is suppressed by encoding the setting of a PLL. However, in these inventions, the number of set states of the PLL, that is, the number of combinations of frequency-division ratios for determining the output frequency is limited in order to compress the amount of data transmitted through the encoding. The limitation of the combinations of the frequency-division ratios makes it difficult to make clocks conform to each other with a high level of accuracy. Accordingly, even when the technique disclosed in JP-A-05-284021 or JP-A-2006-5489 is applied, it is difficult to realize an oscillator suitable for network synchronization.
In addition, an oscillator disclosed in JP-A-2013-98872 adjusts an output frequency by setting a ratio with respect to a nominal frequency via a serial interface. The oscillator disclosed in JP-A-2013-98872 can set a variable width in accordance with a range of use and can set a frequency in the variable width with a high level of accuracy, as compared with the inventions disclosed in JP-A-05-284021 and JP-A-2006-5489. However, for example, it is assumed that the adjustment of the frequency is executed during the supply of power (paragraph 0029 of JP-A-2013-98872), and thus it is difficult to continuously change the frequency. For example, a second register storing a ratio with respect to a nominal frequency is divided into three addresses in order to respond to a request for setting a frequency with a high level of accuracy (FIG. 4 of JP-A-2013-98872). In order to change the frequency, it is necessary to rewrite all pieces of information of the three addresses, which results in a decrease in the modulation bandwidth.